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Mare reproductive loss syndrome returns, but losses are fewer

Posted July 1, 2002

Evidence further suggests that eastern tent caterpillar is culprit

This year as spring approached, the equine industry waited with bated breath, hoping that the mare reproductive loss syndrome that gripped the industry in 2001, killing hundreds of foals and causing thousands of spontaneous abortions, would not return. But it did, albeit losses were fewer.

Statistics indicate that between April 28 and May 25, 270 aborted equine fetuses were submitted for examination to the Livestock Disease Diagnostic Center in 2002, compared with 495 in 2001. The average for this time span from 1996 through 2000 was roughly 100 abortions. According to Dr. David Powell, a veterinary epidemiologist at the University of Kentucky's Gluck Equine Research Center, 160 of the submissions during this 2002 time period were caused by MRLS. Two other cases were identified by June 1.

Although scientists cannot definitively say what is causing the syndrome, mounting evidence suggests that the eastern tent caterpillar is to blame. The finger- pointing is based both on observations in the field as well as on studies at the University of Kentucky.

"Cases have only been reported on those farms or those pastures in the presence of the eastern tent caterpillar," said Dr. Powell. "On farms that had problems last year, but do not have the presence of caterpillars this year, they have remained, at the moment, completely free of the problem."

Several experiments at the University of Kentucky also implicate the wormlike creature. In one experiment, 29 mares that were between 40 and 100 days pregnant were divided into groups; 10 were exposed to caterpillars, nine were exposed to frass (caterpillar waste), and 10 were exposed to neither. Researchers found that three of the control horses aborted, while seven of each of the other groups aborted.

Scientists suggest caterpillars are to blame for the high abortion rates among horses in the two exposed groups. They also fault the control horses' abortions on the multifooted larva, since some of them managed to gain access to the supposedly pest-free areas. Walter Barney, a research analyst at the University of Kentucky, says that after they moved the control horses farther away from the other two plots, they did not lose any additional fetuses in the control group.

Scientists speculate that perhaps a mold, virus, or bacterium from the caterpillar is being transmitted to the horses. Others think that the setae or hairs of the wormlike pest may cause an allergic reaction.

Dr. Powell emphasizes that scientists have not eliminated other possibilities besides the caterpillar, including factors that mimic hormones (anti-estrogens), toxins associated with pasture grasses, yeasts, minerals, and molds. "All we can say is that we may have identified the source and it doesn't tell us the cause, so we still have a great deal of work to do," he said.

MRLS first reared its head in April of 2001, devastating the Thoroughbred industry. Mares in central Kentucky aborted early- and late-term or gave birth to stillborn and weakened foals by the hundreds. A study by the University of Louisville estimated 2001 losses at $336 million. At press time, economic losses for 2002 had not been estimated but were expected to be substantially less.

In October 2001, the University of Kentucky released "A Farm Contingency Plan for MRLS Risk Reduction for Kentucky Horse Farms." The plan offered suggestions to breeders on how to minimize or eliminate exposure of pregnant mares to eastern tent caterpillars. Adherence to this plan by some farms may explain why the syndrome did not devastate the industry as it did last year.

"There have been some various management practices that were put into place this year, and it is probably attributable to them that we have had fewer losses," said David Switzer, executive director of the Kentucky Thoroughbred Association. He says that the association has just set up a non-profit fund for MRLS research.

In the coming months, scientists will continue to research the cause of MRLS and make plans for next year's breeding season. "We will be evaluating, over the next few months, the information we have gained and, based on that, decide which particular direction we need to pursue, both in terms of establishing the cause and developing particular methods to control and eliminate [the syndrome]," Dr. Powell said.